The first objective is to elucidate the function of the mammalian renal pelvis under various physiological conditions and in mammals with different types of pelvis. The peristaltic movement of the pelvic wall and renal papilla under various physiological conditions will be observed and recorded on film, in the anesthetized, operated animal where the urine is made visible by the injection of lissamine green. Changes in composition of ureteral urine resulting from contact with the inner and outer medulla of the kidney will be studied by micropuncture of the ureter and by when the urine bathes the papilla and when it does not. Structure and permeability characteristics of the epithelium lining the renal pelvis will be studied by autoradiography, electronmicroscopy and by injections with various labelled solutes. The second objective is to study the mechanism of fluid secretion in fish tubules as well as the specific mechanisms for divalent ion secretion. Micropuncture and electronmicroscopy of the elasmobranch renal tubule will be continued. Fluid secretion and divalent ion secretion will be studied in isolated fish tubules in vitro. Cell volume regulation in vivo will be studied in mammalian renal medulla and in tissues from euryhaline fishes. Extra- and intracellular concentrations will be measured using polyethylene glycol as a marker for extracellular fluid. Intracellular water content will be estimated from the measurements of total water content in the tissue and the extracellular space.